clip_on documentation note/warning

This try to fix the comment on PR https://github.com/matplotlib/matplotlib/pull/2837

Warning:
I didn't even knew the `clip_on` existence and I'm not sure I am describing the results correctly. 
I am just paraphrasing @tacaswell from https://stackoverflow.com/questions/21615285/how-to-not-cut-flatten-lines-when-moving-the-spines-in-matplotlib/21617349#21617349